Ocean thermal energy conversion (OTEC) is a system to convert the thermal energy stored as the vertical temperature gradient in the ocean into the electricity. Due to the low available temperature difference for heat engines, heat exchangers are the key component to maximize the available work in heat engine. In general, the performance of heat exchanger is individually evaluated as the heat transfer performance and the pressure drop, and the relation of the performance and the effectiveness for achieving the net power has not been clarified yet. This research proposes the comprehensive heat exchanger performance index based on finite-time thermodynamics for designs and developments in plate heat exchangers. The proposed performance evaluation index is applied to existing plate heat exchangers, and the sensitivities of the index to the heat source temperature is confirmed.